Photos not sending through iMessage when on WiFi?

I have an iPhone 4S running iOS6 (although I was having this issue pre-iOS6). My iPhone sends iMessages, so long as they are text, fine, delivers read receipts, everything fine when I'm on WiFi. It used to send photos on all the WiFi connections I typically use (home, campus, favorite coffee shop, etc.) but about a week prior to iOS 6 it decided it would no longer send photos via iMessage. When I want to send a photo I have to turn off my wireless, and send the photo on data. Then I turn wireless right back on and resume sending texts via iMessage. I have no issues with other WiFi functions - web browsing, etc. Is unaffected.
SMS and MMS are on. My iMessaging is directly linked to my phone number, not my Apple ID, if that makes a difference?

Hoping someone can help with this, especially since it appears its not my wireless signal thats the problem as my iPhone has decided to do this over multiple different wireless signals it used to not have an issue with.

Have been playing around at home for a few weeks now, I have an ipad2, and qty 2x iPhone5's NONE of them will send IMessages or Photostream desnt work either on my home wifi setup but work fine on any other wifi's!

I have reset my ipad2 back to factory defaults and I even tried setting up a NEW iTunes account but neither of these things worked.

So as a last resort i have just put in a new ADSL2 (Melbourne, Australia) modem and wifi that I had sitting around, both different brands and have gotten it working!

I can now send iMessages with pictures on wifi and my photo stream has started working again. I can't work out why my old setup doesn't work as I tried factory restoring the modem and nothing.

Old hardware;
- dlink DSL-502T (ADSL2 modem)
- dlink DIR-600 (wifi hotspot)

New hardware;
- Linksys AM300 (ADSL2 modem)
- Sonicwall sonicpoint G (wifi hotspot)

Going to see if i can get the newer dlink wifi to work with my Linksys modem next.